The prior art teaches equipping vehicles with “variable displacement,” “displacement on demand,” or “multiple displacement” internal combustion engines in which one or more cylinders may be selectively “deactivated” or “suppressed,” for example, to improve vehicle fuel economy when operating under relatively low-load conditions. Typically, the cylinders are deactivated through use of deactivatable valve train components, such as deactivating valve lifters as disclosed in U.S. patent publication no. U.S. 2004/0244751 A1, in which a supply of pressurized engine oil is selectively delivered from an engine oil gallery to a deactivatable valve lifter through operation of a solenoid valve under the control of an engine control module. Preferably, the engine control module operates the solenoid valve such that the lifter's locking pins are moved between their respective locked and unlocked positions as the lifter's cam lies on the base circle of its corresponding cam surface, thereby minimizing lifter wear and noise. Thus, the triggering of the oil control solenoids is preferably synchronized to either the crankshaft in a pushrod engine, or the cam shaft in an overhead cam engine.
It is also known that, at each engine speed, there is a range of potential solenoid trigger points that produce a proper sequencing of the deactivatable valve train components, and that the viscosity of the oil supplied to the oil gallery has a significant impact on the amount of time required for deactivation, as a more viscous oil will drain more slowly through the solenoid's and/or the actuator's drain passages.
Because oil viscosity increases with decreasing engine oil temperature, the prior art teaches delaying the enablement of engine displacement mode transitions until the engine oil is sure to be warm. Where the additional expense of an oil temperature sensor on the engine has been avoided and, hence, oil temperature cannot be directly detected, the prior art teaches enabling actuator operation for displacement transitions only after a predetermined engine run time after start-up has occurred. However, this prior art approach necessarily prevents early transitions to a more fuel efficient partial-displacement engine operating mode when the instantaneous oil viscosity is otherwise suitable for actuator operation before the timer has run out, as is likely to occur, for example, when restarting the engine after a short engine run-time that does not itself exceed the predetermined minimum run-time.